Wearing tight underwear can lead to the decrease in size of your penis. In 1745 a custom was established where men would wear tight underwear so that their penises would not bulge, since back then It was rude and disrespectful to have a long penis.

No. No logical reason it would, as guys with really tight underwear can't leave their penis in when fully erect- not enough room. And being constricting when flaccid has nothing to do with determining erect size.